Human CD200 protein is a type 1a transmembrane glycoprotein that is normally expressed on thymocytes (e.g., T cells and B cells), neurons, and endothelial cells. Through engagement with its cognate receptor, CD200R, CD200 protein transduces an immunoregulatory signal that can suppress T-cell-mediated immune responses. CD200 knockout animal studies as well as experiments using antagonist anti-CD200 antibodies and recombinant CD200-Fc fusion proteins have demonstrated that CD200 protein is an immunosuppressive agent in autoimmune disorder and during transplantation. See, e.g., Hoek et al. (2000) Science 290:1768-1771 and Gorczynski et al. (1999) J Immunol 163:1654-1660. The interaction between CD200 and CD200R results in altered cytokine profiles and promotes a TH2 T cell response over a TH1 response. (See, e.g., Kretz-Rommel (2007) J Immunol 178:5595-5605.)